Many vehicles, in particular automobiles, are equipped with occupant detection devices so occupant protection devices such as airbags can be enabled or disabled based on who or what is occupying a seat in the vehicle. For example, if a seat is empty, or occupied by a small child, it may be desirable to reduce the deployment force or disable an airbag for that seat to prevent airbag deployment if the vehicle is involved in a collision. One way to estimate the size of a seat occupant is to equip the seat with a device that estimates occupant weight. One such device has a fluid filled bladder placed in the seat cushion, and a pressure sensor. The occupant weight is estimated by sensing a fluid pressure of the fluid in the bladder.
A typical configuration of such a weight sensing device is shown in FIG. 1. A bladder 10 is filled with a fluid and is positioned in a seat cushion 12. Fluid pressure in the bladder 10 is coupled to a remote pressure sensor 14 by a hose 16 that is also filled with the fluid. The remote pressure sensor 14 may be attached to the seat cushion 12, or attached to the vehicle. When a person sits on the seat cushion 12, a change in fluid pressure in the bladder 10 is sensed by the pressure sensor 14 because of the fluidic coupling through the hose 16. A pressure signal from the pressure sensor 14 is processed by other electronics typically co-located with the pressure sensor 14 to output a weight estimate or seat-occupied signal to the vehicle over a wire 18. This weight estimate or seat-occupied signal is used by other systems in the vehicle to enable or disable occupant protection devices of the vehicle. However, it has been discovered that the fluid pressure in the bladder may not be precisely the same as the fluid pressure sensed by the pressure sensor since other factors may influence the relationship between the weight of the occupant and the pressure sensor reading.
One discovery was that changes in seat orientation or inclination angle relative to the vehicle and/or the direction of gravity may change the effective vertical separation between a point comparable to the center of gravity of the bladder 10 and the pressure sensor 14. The vertical separation creates a pressure difference between fluid in the bladder 10 and fluid at the pressure sensor 14 commonly known as head pressure. If the vertical separation is constant, then the head pressure is generally constant and so the effects can be accounted for when estimating occupant weight. However if the vertical separation changes because, for example, the inclination angle of the seat is changed, then the head pressure will change and could effect the accuracy of the occupant weight estimate.
It was also discovered that vehicle dynamics such as longitudinal acceleration caused by braking or accelerating the vehicle, and lateral acceleration caused by the vehicle turning could dynamically change the fluid pressure in the hose routed to the pressure sensor. While not subscribing to any particular theory, the effect appears to be linked to an effective horizontal separation between a point corresponding to the center of gravity of the bladder 10 and the pressure sensor 14. It should be appreciate that the degree to which vehicle dynamics affect a pressure difference between the bladder center of gravity and the pressure sensor depends on the length and routing of the hose.
Another discovery was that the mechanical properties of the material used to form the bladder changed with temperature. At reduced temperatures the bladder materials became stiffer and less viscous thus becoming less responsive to the occupant weight being applied. According to one theory, the bladder material becomes stiffer at reduced temperatures and so a variable portion of the occupant weight is supported by the bladder structure itself, and so a portion of the occupant weight may not influence the fluid pressure. The prior art attempted to correct this problem by including a temperature sensor in the remote pressure sensor. However because some seats are equipped with heating elements for warming the occupant in cold weather, the temperature measured at the remote pressure sensor did not always correspond with the temperature of the bladder.